The chief of this class is the Melanite, sometimes dull, yet often vitreous; it is mostly found in volcanic rocks, such as tuff; this variety is very popular with jewellers for mourning ornaments, for as it is a beautiful velvet-black in colour and quite opaque, it is pre-eminent for this purpose, being considerably less brittle than jet, though heavier.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin